1. Field of the Invention
The present invention relates to a motor-driven hydraulic pump (hereinafter referred to as "motor-driven pump") and a driving coupler used therein to transmit a driving force from an input shaft to an output shaft, wherein the pump generates hydraulic pressure for operating hydraulic assist devices such as a steer-assisting hydraulic cylinder and an automatic transmission.
2. Description of Related Art
Motor vehicles, especially road vehicles, are commonly equipped with hydraulic assist devices such as a steer-assisting hydraulic cylinder and an automatic transmission so as to maintain an optimum engine speed and a comfortable road running. These hydraulic assist devices are operated by pumps such as vane pumps and gear pumps, which are driven by the engine of the vehicle. In operating the steer-assisting hydraulic cylinder, the pump is operated at varying speeds by reference to the road speeds of the vehicle. For example, it will be operated at high speed when the vehicle runs at low speed or is brought to a stop where a large steer-assisting force is required, and it will be operated at low speed when the vehicle runs at high speed where little steer-assisting force is required. However, the engine of the vehicle is difficult to cope with such frequent changes in the operating speeds of the pump. In order to meet these demands, an extra electric motor whose outputs are readily adjusted is used for the hydraulic pump independently of the engine of the vehicle. This solution is proposed and disclosed in Japanese Patent Publication (allowed) No. 3-15592. It will be briefly described with reference to FIG. 12:
The illustrated motor-driven pump is provided with an electric motor (M) and a pump housing (B) including a support portion (C) which is provided with an oil outlet path (E) through which a working oil is forced out. The electric motor (M) includes a motor shaft (input shaft) (m). The pump housing (B) includes a pump shaft (output shaft) (a) coupled to the motor shaft (m), and also houses a intake chamber (not shown) in which a driving gear (A) driven by the motor shaft (m) and a follower gear (not shown) are mutually in mesh, and deliver a working oil to an outlet chamber (not shown) communicating with the oil outlet path (E).
In recent years, electric vehicles (EV) are developed so as to avoid environmental contamination due to exhaust gases. Electric vehicles are driven by an electric motor instead of an oil engine. They are also equipped with a steer-assisting hydraulic devices operated by a motor-driven pump.
In either case where an electric motor is used to operate the pump, the problem is the limited accommodation space in the vehicle. To solve this problem, the pump and the electric motor are compactly combined or unified as shown in the Japanese Publication No. 3-15592 referred to above where a conventional mortise-tenon connection is used to couple the motor shaft and the pump shaft as shown in FIG. 13.
Referring to FIG. 13, an input shaft (motor shaft) (m) and an output shaft (pump shaft) (a) are coupled to each other by means of a driving coupler (hereinafter referred to as "coupler") (J). The coupler (J) includes a first mortise J1 and a second mortise J2 on opposite ends. The input shaft (m) includes a tenon m1, and the output shaft 4 includes a tenon al. The tenons m1 and al are inserted in the mortises J1 and J2, respectively, thereby coupling the motor shaft (m) to the pump shaft (a) such that a torque is transmitted through this mortise-tenon connection. As shown in FIG. 13, the rectangular mortises J1 and J2 are formed like a cross such that a torque is effectively transmitted from the motor shaft (m) to the pump shaft (a) through the coupler (J).
The mortise-tenon connection advantageously ensures the coaxial alignment of the input shaft (m) and output shaft (a). An alternative embodiment is disclosed in Japanese Utility Model Laid-Open Specification No. 5-58882, the disclosure of which is herein incorporated by reference; briefly, the tenons ml and al are provided in the coupler J, and the mortises J1 and J2 are provided in the motor shaft (m) and the pump shaft (a).
The known motor-driven pump described above has an oil reservoir defined by a space between the teeth of the driving gears (A) and the follower gears and the inside wall of the pump housing (B). The oil is intermittently delivered every time each oil reservoir is opened to the outlet chamber in the pump housing (B). The problem of this system is the pulsation occurring in the flow of oil which causes the driven shaft (pump shaft) (a) to vibrate.
The coupler (J) is normally made of metal but a metal coupler is likely to cause noise in transmitting a torque from the input shaft (m) to the output shaft (a). Sometimes the noise is so harsh that the driver misunderstands that anything is wrong with his or her car.
Researches have found out that the noise is caused by the pump shaft (a) vibrating in accordance with pulsation occurring in the oil flow within the pump housing (B), and rattling against the coupler (J). To avoid this noise problem plastics-made couplers are proposed instead of metal-made couplers. A plastics-made coupler (J) can effectively absorb vibration but is too fragile to transmit a large torque and is liable to fracture under a large load. To withstand large loads, the plastics-made coupler requires a large size of sufficient thickness. Such large and thick couplers may present other disadvantages.